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CRAFT BEER CORNER: Yeast's Role In Fermentation

By Sean Smith
Gazette Columnist
It has been said that “brewers make wort, and yeast makes beer.”
Yeast is the “magic” ingredient that converts sugary sweet wort into ethanol, carbon dioxide and a host of other compounds that lend flavor and aroma to beer. Without an understanding of yeast’s role in fermentation, beer making truly is a mysterious process.

But Louis Pasteur’s observations changed everything. Pasteur brought fermentation into the modern world and for the first time brewers gained a much deeper understanding of how their product was made. Brewing science revealed that the yeast responsible for beer was a variant of the genus Saccharomyces (literally “sugar fungus”) that includes both baking and brewers yeasts. Modern biological science has further enhanced brewing, allowing brewers to isolate specific variants of yeast within the genus and create a means for storing these pure cultures, giving brewers complete control over the fermentation medium used in their beer.
Until recently, ale yeasts (Saccharomyces cerevisiae) were most commonly used in craft beer production. Known as top fermenting because they rise to the surface during fermentation, creating a foam brewers call krausen, ale yeasts generally ferment at warmer temperatures (between 62˚-75˚f) and create beer with readily noticeable flavors from the byproducts of yeast’s metabolism, respiration and reproduction.
John Palmer in his book “How to Brew” offers a fantastic primer on the flavors in ale yeast, but put simply, they contribute ethanol (well duh), CO2, acetaldehyde (green apple aroma), diacetyl (taste or aroma of butter or butterscotch), DMS, (sweet corn), esters (bananas, strawberries, or other fruit), phenolics (medicine, plastic, Band-Aids, smoke or cloves), fusel alcohols (reminiscent of acetone or solvents), sulfur volatiles (rotten eggs or burnt matches), as well as other compounds.
To be honest, none of these sound like something one wants to drink and in fact if the beer is improperly fermented these compounds at elevated levels can reveal poor brewing practices and a lack of sanitation. But when the brewer does his job, these flavor compounds are subtle, and when balanced with the residual malt sugars and hop bitterness create good craft beer.
Here again the brewer’s control over the choice of yeast results in different amounts of these flavor compounds and are often style specific. Pale ales, for instance, are often fermented with a yeast strain that offers low to moderate esters and often a bit of sulfur. Amber ales or browns and porters, shouldn’t show any sulfur or diacetyl, but can show low to moderate esters.
Lager yeasts (Saccharomyces pastorianus) ferment at relatively cold temperatures (44˚-55˚f) staying at the bottom of the fermentation vessels and taking their time to ferment out. The results are clean beers often described as crisp tasting. They lack the fruity esters, diacetyl and fusel alcohol flavors of ales, but highlight low levels of DMS and sometimes sulfur volatiles. The clean flavor profiles and the expense of cold fermentation make them incredibly difficult beers to produce well. They are, I think, the ultimate test of a brewer’s skill and when done well are delicious and quite refreshing.
In the past decade, American brewers also have been experimenting with wild yeasts, open fermentation, or the introduction of a variant of Brettanomyces, another genus of yeast that nearly all brewers save Belgian Monks used to avoid. With wild yeast, the brewer has no control over the fermentation byproducts; the yeast that’s in the air spontaneously ferments the beer, resulting in flavor profiles that vary from batch to batch. Others who want the souring and acetic acid effects of wild yeasts turn to domesticated strains of Brettanomyces, or “Brett,” used to produce lambic and sour style ales common in Belgium, but more and more popular with American brewers.
“Brett” or wild yeast beers are sour and can best be described as funky. They are super complex and many believe them to be an acquired taste. I love them, and find them, especially sour varieties, fantastically refreshing on hot summer days. Russian River Brewing’s Supplication, Depuration Temptation, and Sanctification, or New Belgium’s La Folie series offer good and approachable examples of Brett-fermented beers that are commonly available in Southern California.
Like hops then, yeast is used not only to provide alcohol and CO2 but also to bring important flavor compounds and aromas to beer, adding even more to the brewer’s spice cabinet. In this way each brewer makes his or her own beer.
It’s now time to put all of this knowledge together go grab your favorite Pilsner, Pale Ale, IPA and dark beer. Try them in a vertical tasting (light to dark, but usually ending with IPA or Imperial Stouts) and see if you can pick out the individual ingredients and the flavors we’ve been discussing. If you can, you’ve passed beer 101, just drank some delicious beer, and are now well on your way to becoming a beer geek. Congratulations!
Sean Smith is an award winning homebrewer, historian and writer. You can find more of his writing at www.JustAnotherBeerBlog.com